Global navigation satellite systems (GNSS) are networks of geostationary satellites for geo-spatial positioning using time of arrival (ToA) measurements from line of sight (LoS) radio communications with satellites to calculate position to a high level of accuracy, usually several meters, and simultaneously calculate local time to high precision. The Global Positioning System (GPS) is a GNSS developed and maintained by the US Department of Defense (DoD); other GNSS include GLONASS (Russia), BeiDou (China), GALILEO (Europe), and IRNSS (India). GPS specifically is divided into two classes: SPS, for civilian use, and PPS, for military use which uses two frequencies for ionospheric correction. GPS and GNSS have found myriad uses in civilian, commercial, and military applications for locating and tracking people, goods, and physical capital. However, while standard in many consumer products, including smartphones and automobile navigation systems, GNSS is ineffective at locating in indoor or in urban environments, where the radio signals from the GNSS satellites are blocked by intervening metal or dielectric structures, such as roofs, walls, and windows. Furthermore, implementations of GNSS require LoS radio communications with at least four GNSS satellites, limiting the geographic availability in developing GNSS networks, such as BeiDou and IRNSS, and locations where multipath propagation due to reflections of electromagnetic signals from structures is an issue, such as in urban canyons or indoor environments.
A number of technologies, originally developed for mitigating multipath propagation in radio direction finding applications, have found use in GPS. Since GPS signals are based on sky wave propagation, ground wave attenuation techniques effectively mitigate multipath reflection. An example is choke-ring antennas, developed at NASA Jet Propulsion Laboratory (JPL) and currently licensed by patent holders to Trimble Navigation and Magellan Professional Products for use in GPS receivers.
Radio direction finding (RDF), commonly used in aircraft and marine navigation, such as Decca Navigator Systems and LORAN, in civilian and military applications, as well as amateur radio, is a series of methods for determining the direction or bearing of a radio frequency (RF) transmitter. RDF may be used for determining the location of an RF transmitter, a process called radiolocation, using multilateration (MLAT), based on time difference of arrival or frequency difference of arrival (TDoA/FDoA), or multiangulation, based on angle-of-arrival (AoA). AoA may be determined using directional antennas, such Adcock, Watson-Watt and associated signal processing techniques such as the Butler Matrix, or by measuring phase differences between individual elements in antenna arrays, such as Correlation Interferometry. TDoA/FDoA requires synchronization to a common time base, which is conventionally an absolute time reference with a high level of timing accuracy, such as an atomic clock.
An active area of research and development for market applications are so-called indoor positioning systems (IPS), local positioning system (LPS), and real-time locating systems (RTLS), which use information from a variety of sensors, such as Wi-Fi, Bluetooth, magnetic positioning, infrared, motion sensing, acoustic signals, inertial measurement, LIDAR, and machine vision, to locate physical objects or personnel in indoor environments or urban canyons where traditional GPS is ineffective. These technologies may be complemented with ToA synchronization, such as from using pseudolites or self-calibrating pseudolite arrays (SCPAs), with positional accuracy under 1 meter in some cases. A pseudolite is typically a local, ground-based transceiver used as an alternative to GPS.
IPS may be implemented at choke points, as a dense network of short-range sensors, or long-range sensors based on AoA, ToA, or received signal strength indication (RSSI) The feasibility and cost-effectiveness of IPS has been increasing with the current and future trend towards larger numbers of indoor antennas at access points for cellular and wireless communications, as in the case of multiple-input and multiple-output (MIMO). This has been driven by the demand for increased coverage indoors and the emerging 5G telecommunications network standard, which will have smaller cell sizes due to the use of higher transmission frequencies with shorter propagation ranges, with the goal of spectrum reuse, and networking of buildings, vehicles, and other equipment for Web access, sometimes informally referred to as the “Internet of Things” or IoT.
Several commercial solutions exist for mobile phone tracking. These are based on tracking of GPS-capable smartphones, Wi-Fi-capable smartphones or feature phones, and cellular positioning. The US government specifies a worst case pseudorange accuracy of 7.8 m at 95% confidence level for GPS. For a 3G iPhone, the positional accuracy for these three techniques has been established at ˜8 m, ˜74 m, and ˜600 m, respectively. External GPS hardware may be used with smartphones and feature phones for additional positioning accuracy, such as XGPS150A, with a positional accuracy of ˜2.5 m.
Wi-Fi positioning is currently a developing technology for tracking, and is based on signal tracking of transmissions from wireless devices, wireless access points (WAPs), and routers. Packet monitoring can provide the MAC address of the transmitting device and signal strength through received signal strength indication (RSSI), which may be used for locating the device. Wi-Fi positioning has a propagation range of ˜100 m, and at least 1-5 m positional accuracy. This technique is most effective in urban environments with a large number of signals. Wi-Fi positioning has been implemented in systems based on the range of the transmitting device from a receiver or AoA with antenna arrays, which may be implemented on commodity wireless access points (WAPs) by taking advantage of existing MIMO capabilities and developing various additional signal processing capabilities into software, such as multiple signal classification (MUSIC).
The drawbacks with these existing techniques are that they either have extremely limited positional accuracy and coverage indoors, such as GPS/GNSS, are applicable for only certain communication protocols like Wi-Fi positioning, are not robust to data collection artifacts such as machine vision using cameras, or require extensive hardware infrastructures to support like machine vision and Wi-Fi positioning. The latter consideration is especially relevant, as it presents a limiting factor to the market adoption of a particular technology for tracking purposes due to cost and implementation barriers. Furthermore, the accuracy requirements are more stringent for indoor positioning, where it is often desirable to achieve accuracy on the 1 meter scale or smaller to provide location information within a single room in a building.